There are many routes to the £300 a week you are after. People make money online in all sorts of ways and you have to find the way thats right for you because you will be spending a lot of time with these sites.
Minisites seem a good idea. They are generally 'build and forget' which means they generate revenue without you doing much to them. Be aware that, depending on the niche you might end up having to build 100+ sites. Also, it takes time to organise the content, do the
SEO stuff and get backlinks. From what I have read, £20 per month seems to be more realistic and you might have to build 150 sites to get 100 that generate £20 consistently. Niche selection and high paying keywords are fairly important as well.
Thats why some people advocate the creation of a smaller number of 'authority sites'. You can dominate a niche, especially if the sites appeal to a similar group/ niche. Your site becomes the site people go to when they want to get the latest news/ gossip whatever.
Either way if you are going for a simple ad revenue business model you may end up having to produce 1000 + pages, and probably much more, before you see decent money coming in.
Here is an interesting thread from another forum, (from 2006 but still valid) that readers of this thread might find of interest.
My Bank Statement - Monthly Earnings from AM - SitePoint Forums
The point is people are making money both ways.................